1. Field of the Invention
The present invention relates to a control system for actuating vehicle air-bags, and more particularly to a system for controlling the firing time of an inflater of an air-bag.
2. Description of the Prior Art
For ensuring the safety of car occupants from the shock occurring at the time of a vehicle collision, there has been used an air-bag control system, in which firing current is supplied to an electrically fired actuator to cause gas to be produced by a chemical reaction by igniting gunpowder so that the air-bag is inflated by the gas. Accordingly, for the control system of this type it is necessary to detect when a collision has occurred as soon as possible, and to ignite the gunpowder without fail when a collision has occurred.
To meet these requirements, in the conventional air-bag control system, the output signal from an acceleration sensor is integrated when the output level of the acceleration sensor mounted on the vehicle exceeds a predetermined level to obtain information concerning the vehicle speed thereafter, and it is determined that the vehicle has collided with an obstruction when the decrease in the vehicle speed reaches a prescribed level and the gunpowder is then ignited. Such a conventional system is disclosed in, for example, U.S. Pat. No. 3,911,391.
It is generally said that a safety device can effectively protect the car occupants only if it can limit the maximum displacement of the occupants, especially their heads, to within fifteen to twenty centimeters at the time of collision. Accordingly, considering that it takes approximately thirty milliseconds from the time of the actuation of an electrically fired air-bag actuator to the time of the completion of the expansion thereof, it is necessary to decide whether or not a collision has occurred within twenty to twenty-five milliseconds after it happens. The conventional control apparatus of this kind is of course designed in consideration of this fact to perform the appropriate triggering operation for the electrically or mechanically fired actuator.
On the other hand, the rate of pressure increase in the air-bag occurring after the triggering of the inflater depends largely upon the ambient temperature at that time. For example, at an ambient temperature of -40(.degree.C.), it will be the 10(m sec) or more longer to obtain the required inflation of the air-bag than at room temperature. Consequently, it is not always possible to ensure the appropriate triggering control of the air-bag in the case where the triggering timing of the air-bag is determined in the way described above on the basis of only the magnitude of the acceleration occurring at the time of vehicle collision, so that it may be difficult to ensure the safety of the car occupants in the case where a collision occurs at a low temperature.